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Broadcast "Wave Propagation"

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Electromagnetic field

Electrical field (EMF)

There is an electrical field (EMF) between the two points that have different electrical charge.

This link is to Google the topic on electric fields, for further studies

As you can se in the picture below, there is a field between the two point that are directed from the positive to negative point. This field is electrical and can be seen as acceleration works on a mass.

Magnetical field

Magnetical field is something that occurs around a cable when there are going current in it, or when you have magnetic differences in iron for instens. You can charge a screwdriver and then pic up screws with it, this is also because of magnetic field.

The magnetic direction of the field can be determend by the "right hand grip" -rule. It says that when you have your right han around a cable in the current direction, the magnetic field goes in the same direction as your fingers.

Electromagnetic field

When it comes to the electromagnetic waves in free space, the two forces are related to eachother and to the speed of light "c" (c = 299.792.458 1.2 m/s.). the two forces have a 90 degree angle between them, and they vary with the same phase. The amlitude of the two forces follows another linear.

The relationship between the wavelength λ and the frequency f is given by the formula:

c = λ * f

For further investigation of the topic, se this great link, http://www.lightandmatter.com/html_books/0sn/ch11/ch11.html

Elektromagnetic spectrum

The electromagnetic spectrum doesn't have limits so the picture below is only for the spectrum belonging to information transport over an modulated wave in the part interesting for us in the telecom and broadcast business.

Polarization of ewaves

As we send electromagnetic waves from an antenna, it have a given amplitude, wavelength, and speed. The waves leaves the antenna when the current is zero and the potesial differense between the to ends of the antenna is maximum. We then have an electric field between the ends of the antenna.

Vertical polarisation

Is a linear polarisation. That means that the waves leaves the antenna in straight lines like waves in water when you drop a stone in it. One example is the antenna one the WLAN systems most of us have at home. They've got a small antenna of about 10cm and is straight. They also send out a linear electromagnetic wave. If the antenna is vertically we say that we have a vertically polarized antenna. It is the electric field that is vertical and the horizontal magnetic. So it is the electrical signal that sets the name of the polarity we have.

Horisontal polarisation

Is also a linear polarisation. The difference from vertical polarisation is that the antenna is mounted horisontal. This is the most used polarisation method in regular brodcasting systems like television and FM radio.

Refraction, deflection and reflection of radio waves

Radio waves are electromagnetic waves that are very similar to the electromagnetic waves of light. This means that many of the same consideration to light, can be used for radio waves. This is more properly the higher the frequency is. Visible light ranges from 380nm to 740nm, this is significantly shorter waves than for radio waves, but still many of the same rules applyes.

When the waves hit a different medium it will be bent and reflected. How much is reflected and how much is bendt, depends on the molecular density in the medium it hits. Radio waves is most common from the air, but can also come from water. In the cities the biggest problems are buildings, walls, windows, cars and other physical obstacles. In rural areas it is mainly mountainous, trees and terrain, we must take into account.

it is not always a problem that the waves are influenced by the environment. Many times we are totally dependent on what happens. For example, suppose that you sit in a cabin deep in the woods, with trees on all sides. Wanted to be boring if you could not hear on the radio, watch TV and talk on the cell phone because of this. Many times you may not have coverage either, but what is certain is that a reflection can be enough that you have coverage.

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